The present invention relates to a system for holding a plurality of light beams in predetermined relative positions. The system according to the present invention finds its suitable application to a system using a plurality of light beams obtained from a plurality of light sources, such as an optical disk recording system, a laser beam printer or a laser facsimile.
In the prior art, there are several devices for attaining high light output by aligning a plurality of laser beams into one optical axis and by focusing the same on one point in a recording medium. Of these devices, there is known: a method using a polarizing prism 3 which allows a p-polarized light 1 to transmit therethrough while reflecting an s-polarized light 2, as shown in FIG. 1, by making use of the polarizing characteristics of a laser beam; a method using a mirror filter 6 having its multiple layers coated to allow one beam 4 of two laser beams having different wavelengths to transmit therethrough but to reflect the other beam 5, as shown in FIG. 2; a method using a diffraction grating 7, as shown in FIG. 3; or a method making use of a hologram 8, as shown in FIG. 4.
However, any of the aforementioned methods finds it difficult to register spots on the recording medium, and is susceptible to the effect of external disturbances such as vibrations or changes in temperature, so that even if registration could be made by an adjustment, the inherent defects make it likely that such registration would be unsatisfactory. Thus, it is the current state of the art that the existing methods do not suffice in practice.
In a system such as a laser printer or a facsimile for scanning with light, optical scanning is conducted by moving a rotating polygonal mirror or a galvanomirror at high speed. To achieve high speed and high resolution, it is necessary to speed up the operation to a very high level, but this level has its limitations.
To cope with these limitations, current practices to reduce the need for high operating speed of the optical deflector of a rotating polygonal mirror by conducting simultaneous scanning operations with a plurality of light spots obtained from a plurality of light sources.
These light sources are generally of two types: (1) a semiconductor laser array or light emitting diode array (2) a plurality of light source elements.
When these light source elements are used, the light source elements must be accurately positioned relative to each other. However, it sometimes happens that adjacent spots overlap or are spaced too far apart, with unsatisfactory results in either case.